1. Field of the Invention
The present invention relates to a demagnetization detection device and a demagnetization detection method thereof; and more particularly, the demagnetization detection device and the demagnetization detection method thereof of the present invention determines whether a testing permanent magnet motor is demagnetized by detecting the difference between the power value of a direct current (DC) power source of a power control unit and a standard power value in real time.
2. Descriptions of the Related Art
Due to the rapid development of material science and technologies, the manufacturing processes of permanent magnets with a high magnetic energy have become increasingly sophisticated, and related industries have also benefited from this development. For example, over recent years, immense efforts have been made in the electromobile industry at home and abroad to develop permanent magnet motors in the hope that by virtue of the superior performance of permanent magnets, the permanent magnet motors can be made to have a higher power density and higher efficiency.
When a permanent magnet motor is used for a driving device of an electromobile, a full-range flux-weakening control is usually adopted for the permanent magnet motor to deliver ideal output performance, i.e., to satisfy the requirements of a large torque at low rotation speeds and the ability to operate at a high rotation speed. In other words, through the full-range flux-weakening control, the magnetism of the permanent magnet motor during operation can be adjusted to further control the operations of the driving device of the electromobile at various rotation speeds and torque levels.
However, when the full-range flux-weakening control is adopted for the permanent magnet motor, the permanent magnet of the permanent magnet motor is exposed to a demagnetizing field for a long time, which will cause an accumulation of heat energy in the rotor of the permanent magnet motor due to the iron loss. Once the accumulated heat energy causes the temperature of the permanent magnet to go beyond the limitation of the magnetic flux density—magnetic field strength curve (i.e., the B-H curve) thereof, the permanent magnet would be demagnetized. Furthermore, if the permanent magnet is demagnetized, the magnetic flux density and the coercive field thereof will be weakened abruptly to cause an irreversible demagnetization status; i.e., even after the temperature of the demagnetized permanent magnet decreases later, the magnetic flux density and the coercive field that have been weakened will not restore their original statuses.
Accordingly, once the permanent magnet of the permanent magnet motor is demagnetized, the operational torque of the permanent magnet motor will decrease abruptly to cause abnormal conditions of the electromobile that uses the permanent magnet motor as a driving device. As an example, when the permanent magnet motor is used for an electromobile, the operational torque is determined by a force applied by the driver to the accelerator. In case the permanent magnet of the permanent magnet motor is demagnetized, the actual operational torque of the permanent magnet motor will become smaller than what is expected. Consequently, if the permanent magnet of the permanent magnet motor is demagnetized during the driving process, the driver will be unable to manipulate the electromobile by operating the accelerator as expected, which leads to a dangerous situation.
In the prior art, to avoid overheating of the permanent magnet motor, a temperature sensing element is usually disposed in the stator of the permanent magnet motor to sense the temperature of the permanent magnet motor during operation. However, the temperature sensing element disposed in the stator is only able to measure variations in temperature of the stator but cannot detect the temperature of the rotor accurately, let alone, detect the temperature of the permanent magnet of the permanent magnet motor and whether the permanent magnet is demagnetized. Therefore, the way of determining the temperature of the whole permanent magnet motor by merely detecting the stator's temperature and determining whether the permanent magnet is demagnetized according to the temperature thus detected presents significant errors and great uncertainties.
Accordingly, an urgent need exists in the art to provide a solution capable of accurately and effectively detecting whether a permanent magnet motor is demagnetized.